1. Field of the Invention
The present invention relates generally to an improvement in and relating to an output stage of a solid-state image pick-up device, such as a charge-coupled device (CCD). More specifically, the invention relates to an output stage for a solid-state image pick-up device which is operable even at low voltage.
2. Description of the Background Art
In the past, an image pick-up device, such as a CCD, is provided with a higher power source voltage in comparison with that in other semiconductor devices, such as a memory composed of a plurality of MOS transistors. As a typical output stage construction of the image pick-up device, an inverter circuit or a source follower circuit are employed. In the former case, the inverter circuit comprises a pair of first and second MOS transistors. The first MOS transistor has a gate to receive an input signal. The second transistor has a gate and drain connected to each other for forming an active load. An output signal is output between a drain of the first MOS transistor and the second MOS transistor.
In the inverter circuit as set forth above, the output level may not coincide with the input level. Therefore, when such an inverter circuit is combined with a low-pass filter, a sample/hold circuit and so forth, the source voltage has to be sufficiently high so as to avoid an influence by fluctuation of the output level. In addition, in such an inverter circuit, the circuit design becomes complicated which makes it difficult to design.
On the other hand, the conventional source follower circuit may comprise a pair of first and second MOS transistors. The first MOS transistor receives the input signal at its gate and outputs the output signal from its source. Such source follower circuit has a substantially small gain, e.g. less than or equal to 0 dB. Therefore, when a plurality of source followers are connected to form multi-state source follower circuit, the gain of such circuit becomes unacceptably small.
On the other hand, recently, there is a tendency to require an image pick-up device which is active at a lower power source voltage. If the image pick-up device for low power source voltage operation is designed employing the conventional inverter circuit or the source follower circuit, the level margin becomes substantially small to cause malfunction when fluctuation in the DC level occurs.